Working 1/2 day tomorrow...can you re-use milk if they didn't finish it??

My baby is 5 weeks. I'm still on maternity leave, but I will be working 1/2 day tomorrow to fill in for a co-worker. My mom will be watching my son. If he doesn't finish a bottle at one feeding, can it be used for the next feeding? Do I need to refrigerate it in between feedings? Can it be re-heated after the first feeding? It's been SO long since I had to do this with my daughter, I can't remember. Help!